Sulfopolyester polymers are generally referred to polyesters containing ionic sulfonate (SO3−) groups, particularly to those synthesized using a sulfomonomer such as, for example, 5-sodiosulfoisophthalic acid (5-SSIPA or SIP) or dimethyl 5-sodiosulfoisophthalate, as one of the diacids in the polyester compositions. Such sulfopolyester polymers are commercially available from Eastman Chemical Company under the Eastman AQ™ family of polymers. The polymers are linear, amorphous polyesters that can be dispersed in water without the assistance of surfactants or amines. This water dispersibility is attributed to the ionic nature of the sulfonate substituents attached to the polymer chains.
The common Eastman AQ™ polymers are Eastman AQ 38S, AQ 48 ultra, and AQ 55S, which have Tg ranging from 35° C. to 55° C. as denoted by the names of the products. Another product, Eastek™ 1200 polymer, has a higher Tg of about 63° C.; it has applications in printing inks, overprints, and primers for industrial wood coatings. The product is capable of providing excellent water resistance, alcohol resistance, and is fast drying. These properties are also desirable for top coat application in wood coatings; as a result, there have been increased interests in Eastek™ 1200 polymer for wood coating application as a top coat. In addition to sulfonate groups, sulfopolyester polymers typically contain the residue of a hydrophilic polyhydroxyl compound such as ethylene glycol (EG), diethylene glycol (DEG), or polyethylene glycol (PEG) in the polymer chains to further impart water dispersibility. Such EG-based glycols, however, are not preferred for applications in coatings due to their deficiency in hydrolytic stability. Although the water dispersibility of a sulfopolyester polymers without EG-based glycols can be retained by increasing the ratio of the sulfonate groups, it is desirable to keep the sulfonate content low in order to obtain adequate water resistance of the coatings. It is, therefore, an object of this invention to prepare sulfopolyester polymers with low sulfonate content, low EG-based glycol content, yet retain good water dispersibility.
The EG-based glycol components may be replaced by diols that are known to be more hydrolytically stable, such as neopentyl glycol (NPG) and cyclohexane dimethanol (CHDM). Sulfopolyester polymers based on such diols, however, are not as water dispersible as those based on EG, DEG, or PEG. A need therefore exists for a diol component that is not EG-based but is capable of providing good water dispersibility. The advantage of such a solution is to either improve the water dispersibility of such a polymer compared to other non-EG-based glycol modified sulfopolyesters having the same quantity of sulfonate groups, or retain an equivalent level of water dispersibility with a lower amount of hydrophilic groups, thereby improving the water resistance of the coating.